It all depends how many miles you ride. I'm of the opinion that for absolute maximum most totally spanky performance, it should be every 3,000km or so (I believe this is 1,900 miles). However, I normally don't replace it quite that often, and when I was an impoverished university student, I made one chain last over 18,000km!

Either way, I believe it's something that should be measured in terms of distance ridden rather than time.

I'd written a brilliant reply chock-full of useful data but lost it. Here's the deal. Chains wear out at different rates based on how well they are maintained (cleaned & lubed), how the bike is used (road, off-road, recreation or racing), and the user's riding style (pedal masher, spin-city).

You'll need to keep an eye on your chain for the first 500 - 1000 miles to see how fast you're wearing it down to get a feel for about how often you'll need to replace YOUR chains. Sheldon Brown (as always) has a great information page on chain stretch and checking your chain's length if you don't have access to a Park Chain Checker2. http://www.sheldonbrown.com/chains.html

This page, like all of Sheldon's writings, is filled with facinating information. However, what you're looking for is at the bottom of the page.